I think my stages power meter is reading low
 

This week, I've been on two rides. One ride was with a former pro/state champ/local hardass. Another ride was a race earlier today, in a breakaway. I was doing no less work than the other, if not more work. However, when I look at my power data, compared to theirs, there's a maybe 30W discrepancy.. I highly doubt I have a very big aero advantage over them. My stem is slammed pretty low, but everything else on my bike is "normal" (round tube frame, aluminum box rims.. I did wear an aero helmet, but that's a small gain). When I go back to look at the data, my power is mid/high 200s, and theirs are 300+.. I don't get it

https://www.strava.com/activities/1082353954/overview
https://www.strava.com/activities/1082243300/analysis

we were in the same breakaway today. I'm pretty sure I did more work. He was pretty cooked.

Simple -- just take a few minutes and check the slope... oh, wait... :)

Can you put the Stages arm on another crank-based meter? Can you borrow a PowerTap wheel for a day? Can you ride up a steep climb on a calm day and check against predictions, or from prior efforts?

Your meter could be off, or this could be a case of an imbalance getting magnified as the Stages is not measuring power from both legs, or power might have been varied (not something the Stages devices are particularly well suited for), or something else altogether. Maybe THEIR meters are off, too.

Aero helmets can be a sizable gain--unless you've tested (maybe you have) and verified that it is small for you.

I do wish folks would realize how the ability to check and set the slope of a meter is highly desirable, if not essential, for anyone who pays attention to power data over years of training.

My experience with Stages have never been good. I have had them spike to 2000 and read low (different units). In the end I just do not trust their data on an absolute level. My best suggestion is to treat a Stages unit as only comparable to itself it seems they are in general useless to compare to other powermeters.

Your PM could be off, but it's also possible that he just uses more power to go the same speed. Position is by far the biggest aero factor and differences aren't always that obvious. I have a teammate who consistently needs 50w more than me to go the same speed, despite being roughly the same size.

Or your PM could be off.

I'll ask a teammate or something to see if I can test them side to side..

I did do a kickr session the other day with my power meter. It was a trial study (I was the lab rat) where they have me pedal at z1 for an hour under 100C. the power for the kickr was set to 130. The average power for the kickr read maybe 120, and my power meter read an average of 110..

It doesn't bother me too much that my power meter reads lower than others. I was just wondering if this is an issue that others have experienced. So long as I can still do intervals and track my progress accurately, I'm good..

Muscle imbalance maybe?

Didn't that study come out that said Stages sucks?

Maybe your weight is different. Maybe your ability to stay hidden and efficient in the group is different.

Maybe your equipment and aero position are different.

Or, I know it's a long shot but hear me out, the other guy's meter could be reading high.

they did publish a study comparing different power meters, and the stages had the most variability apart from srm and another one that i can't recall. the others weren't very rigorously tested.

anyway, my left:right power distribution is actually around 51:49, according to a couple of trainer interval sessions i've done

Thats easily within a normal L/R imbalance. Doing intervals the other night on my TT Bike (P2Max Meter) at 305W. Typical for me 47/53 L/R so the power was 142/163 for the interval. I think a Stages would have read 284 so about 20W lower (L power X2)?

I have a PT C1, and my L/R is around 50/50, usually 51/49 just like you.

One of the linked activities isn't popping up for me, so I'll throw this out there...is it possible that you are including your zeroes into average power and he isn't?

Keith

FWIW Stages have a lot of "trouble" with temperature changes, among other things. I have seen clients units go 2-5% off kilter (science talk there) with just a couple of degrees change in temp.

i have auto pause set at <1 mph, and for the highlighted segments, we didn't stop. so no, I wasn't recording any more zeros than he was

2-5% off in which direction? is there a correlation? if so, what is it?

Don't get old, you forget things.

I can't recall which direction. Client had two Stages on two bikes and I was seeing a lot of data fluctuation; I was able to meet her at her house and we went through the offset zeroing procedure. Hour or so later we were going to ride, and rechecked. Morning sun was hitting the garage at that point, happened to screen shot the temp from the prior calibration. Temp went up about 5 degrees in the garage, both units showed bad calibration numbers.

Checked later in the ride and they were off again as it got hotter.

Whether there was a linear relationship or a curve, I can't recall. Don't think I did the math on that. But it was a pretty big jump for relatively small ambient temp changes.

FWIW I have seen the same temperature sensitivity issue with Quarqs. SRM's tend to be more stable, as do PTaps. Can't speak to the various pedal units.

Don't know if this is related to the size/design/material of the strain gauges themselves or the surrounding material.

In every case if you're pulling a bike out of a heated or cooled environment I'd suggest letting them sit out in the riding environment for a bit to let the parts equalize.

so we should do zero reset every time the temperature changes? screw that..

btw. I forgot i made my activity private. it should be public now

How do you compare in height and weight to the other rider?

Looking at the Strava analysis, it does appear as if you are coasting a lot more than the other rider. Those do look look like some pretty wicked corners which account for some of the coasting.

You should, in general, with any power meter. I mean, I don't usually, but if you want accurate numbers, you should.

that means you have to leave the bike outside for a while to allow the temp of the device get up to ambient. i guess you can warm up for half an hour, and do a reset before starting the interval or something..

Yeah either way. People do those things. I used to, then I became old and jaded.

https://en.wikipedia.org/wiki/Strain_gauge

scroll down to "variations in temperature"

apparently, there are things you can do to combat this problem. the easiest way is to manufacture the device with a certain type of alloy, such that the changes in resistance attributed to temperature is cancelled out by the change in resistance due to thermal expansion of the material. I assume this is what the power meters use. I want to contact stages to see if this is what they did

How hard is it to zero offset a stages? For my quarq I just have to backpedal like 4 times and for my power2max I have to coast for a few seconds. Easy stuff.

If the data are that important, sell the stages and pay a few extra bucks for a better meter.

zero has to be done while stationary, when no force is put on the arm and it's in the 6 o clock position

i don't care that much. just wanted to get a perspective

it's up to you how you handle it; some devices are much more sensitive to temp swings than others.

specifically: depends on the meter.

one data point: the physical orientation of the strain gauges in SRM is such that changes in material properties due to heat tend to cancel out, so temp swings have minimal to no impact. it is pretty normal for me to take a bike from an uninsulated garage in the winter (temps <=20F) and ride indoors (70F) with no change. also, a normal summer day has overnight temps of 45, 50F and can be 95-100F in direct sunlight.

other meters can and do vary more when temp fluctuates.

maybe just do this for an 'important' workout? for example, if you're doing a 20' test, warm-up, then check ZO before and after that interval.

for a tempo session, who cares?

oh i'm not disciplined enough, and my schedule is typically too scattered to even do a good 20' test.